Alkylation of aromatic compounds with a C2 to C4 olefin and transalkylation of polyalkylaromatic compounds are two common reactions for producing monoalkylated aromatic compounds. Examples of these two reactions that are practiced industrially to produce cumene (isopropylbenzene) are the alkylation of benzene with propylene and the transalkylation of benzene and a diisopropylbenzene (DIPB). The alkylation reaction forms cumene and common byproducts such as DIPBs and triisopropylbenzenes (TIPBs). DIPBs, TIPBs, and some of the higher polyisopropylbenzenes can be readily transalkylated by benzene to produce cumene. Combining alkylation and transalkylation in a process can thus maximize cumene production. Such combinations can have two reactions zones, one for alkylation and the other for transalkylation, or a single reaction zone in which both alkylation and transalkylation occur.
A key operating variable in alkylation and transalkylation reactions to produce a monoalkylated aromatic is the molar ratio of aryl groups per alkyl group. The numerator of this ratio is the number of moles of aryl groups passing through the reaction zone during a specified period of time. The number of moles of aryl groups is the sum of all aryl groups, regardless of the compound in which the aryl group happens to be. In cumene production, for example, one mole of benzene, one mole of cumene, and one mole of DIPB each contribute one mole of aryl group to the sum of aryl groups. The denominator of this ratio is the number of moles of alkyl groups that have the same number of carbon atoms as that of the alkyl group on the desired monoalkylated aromatic and which pass through the reaction zone during the same specified period of time. The number of moles of alkyl groups is the sum of all alkyl and alkenyl groups with the same number of carbon atoms as that of the alkyl group on the desired monoalkylated aromatic, regardless of the compound in which the alkyl or alkyl group happens to be, except that paraffins are not included. In cumene production, the number of moles of propyl groups is the sum of all propyl and propenyl groups, regardless of the compound in which the propyl or propenyl group happens to be, except that paraffins, such as propane, n-butane, isobutane, pentanes, and higher paraffins are excluded from the computation of the number of moles of propyl groups. For example, one mole of propylene and one mole of cumene each contribute one mole of propyl group to the sum of propyl groups, whereas one mole of DIPB contributes two moles of propyl groups and one mole of TIBP contributes three moles of propyl groups. Hexylbenzene and nonylbenzene contribute no moles of propyl groups. Another related operating variable in alkylation reactions to produce a monoalkylated aromatic is the molar ratio of aromatic substrate per alkylating agent, and in cumene production the numerator of this molar ratio is the moles of benzene and the denominator is the moles of propylene.
Many catalysts containing zeolites have been proposed and used for alkylating and transalkylating aromatics. Regardless whether the reaction is alkylation or transalkylation, it is of critical importance that the zeolitic catalyst exhibits not only the capability to initially perform its specified functions, but also that it has the capability to perform them satisfactorily for prolonged periods of time. The analytical terms used in the art to measure how well a particular catalyst performs its intended functions in a particular hydrocarbon reaction environment are activity, selectivity, and stability. And for purposes of discussion here, these terms are conveniently defined for a given charge stock as follows: (1) “activity” is a measure of the catalyst's ability to convert hydrocarbon reactants into products at a specified severity level, where severity level means the conditions used—that is, the temperature, pressure, contact time, concentration of reactants, and presence of diluents such as paraffins; (2) “selectivity” refers to the amount of desired product or products obtained relative to the amount of reactants charged or converted; and (3) “stability” refers to the rate of change with time of the activity and selectivity parameters—obviously, the smaller rate implying the more stable catalyst.
In a process for producing cumene, for example, activity commonly refers to the amount of conversion of propylene that takes place at a specified severity level and is typically measured by olefin content of the alkylation reactor effluent; selectivity refers to the amount of cumene yield, relative to the amount of the propylene consumed that is obtained at the particular activity or severity level; and stability is typically equated to the rate of change with time of activity, as measured by the position of the maximum temperature (due to the exothermic reaction) in the catalyst bed after a suitable interval of time.
However, when a continuous alkylation process is run to produce a constant olefin conversion, the severity level is continuously adjusted by adjusting the conversion temperature in the reaction so that the rate of change of activity finds response in the rate of change of conversion temperatures while changes in the position of the exotherm are customarily taken as indicative of activity stability. Alternatively, additional catalyst may be used to compensate for any activity instability such that the change in position of the exotherm is allowed to proceed through the catalyst bed without changing reaction temperature until such time that olefin breakthrough occurs. At the time of breakthrough, action must be taken to either increase operating temperature to make up for the loss in activity or the catalyst must be regenerated to recover lost activity. Stability is also often typically equated to the rate of change with time of yield, as measured by cumene yield.
Accordingly, a major problem facing workers in this area of the alkylation art is the development of selective catalytic systems that have activity-stability and yield-stability. Zeolites that have been proposed and/or used for alkylating aromatics include beta, UZM-8, and other zeolites including Y, ZSM-5, PSH-3, MCM-22, MCM-36, MCM-49, and MCM-56. In response to the hydrocarbon processing industry's demands for lower molar ratios of aryl groups per alkyl group and more efficient utilization of feed olefins, improved processes for the production of alkylaromatics are sought.